You can 'fake' an html extension, ubb.classic does it. But from an archive/ search engine bot angle, it's not needed - either way (with an .html extension like .classic does it or not, like threads does it) works well with search engines.
That said, it probably wouldn't be difficult to do something like either the 'active topics' link or one of the '24, 48, 7 day' links like at the bottom of the ubbthreads.php page.